The disclosure of Japanese Patent Application No. HEI 10-291298 filed on Oct. 13, 1998 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
This invention relates to a hydrogen generator and to a control method for the generator, which can be connected to fuel cells installed in a vehicle as a driving power source.
2. Description of Related Art
For example, fuel cells are known as devices for converting energy released in a chemical reaction directly into electrical energy. A typical fuel cell has a pair of electrodes that are disposed so as to sandwich an electrolyte member therebetween. An electrochemical reaction occurs when a hydrogen-containing reaction gas contacts a surface of one of the electrodes in the pair of electrodes in an individual fuel cell and an oxygen-containing gas (e.g., air) contacts a surface of the other electrode. By utilizing this electrochemical reaction, fuel cells generate electrical energy.
Hydrogen gas introduced into the individual fuel cell can be supplied from a hydrogen tank. Also, hydrogen can be produced by a reforming apparatus. For example, hydrocarbon raw material, such as methanol or methane, is introduced into the reforming apparatus to produce hydrogen-rich gas, in which the main component of the gas is hydrogen and the rest of the gas is a mixture of carbon monoxide, carbon dioxide, etc. For example, when producing hydrogen-rich gas from methanol, a mixture of methanol and water is evaporated in a vaporizer of the reforming apparatus, and then the vapor of the mixture is reformed into hydrogen-rich gas by a catalyst in the apparatus. After reforming, carbon monoxide contained in the hydrogen-rich gas, which is an undesirable substance for fuel cells, is oxidized into carbon dioxide. Thus, the hydrogen-rich gas in which the carbon monoxide concentration has been reduced is introduced into fuel cells.
A fuel cell system in which external heat is utilized for reforming is disclosed in Japanese Laid-Open Patent Application No. HEI 06-181065, for example. In this system, it is possible to utilize surplus vapor, which is waste heat from a boiler of a thermal power station, to vaporize the raw material, so that energy for producing vapor is saved. Additionally, due to the large surplus vapor, the system can respond flexibly to a change of required load of the fuel cells.
However, a sufficient surplus heat source is not available from the outside of the fuel cell system when the system is utilized as a power source for a vehicle. This is because there are few sources around the fuel cell system, which generate sufficient surplus heat. Accordingly, it is difficult to output a desired power, which is required to fuel cells, in a short time, when a driving condition of the vehicle is rapidly changed. Thus, response speed of actual output power of fuel cells against a change of required power, which is controlled by the speed at which hydrogen is supplied to the fuel cells, depends on the speed at which vapor can be supplied to a vapor reforming apparatus connected to the fuel cells.
Accordingly, it is an object of this invention to provide a hydrogen generator for fuel cells installed in a vehicle as a driving power source, which can readily supply a sufficient amount of hydrogen to fuel cells without an external heat source which generates surplus heat.
In order to achieve this and other objects, in accordance with one aspect of this invention, there is provided a hydrogen generator connected to a fuel cell installed in a vehicle as a driving power source. The hydrogen generator includes a reformer and a vaporizer. The reformer produces hydrogen from vapor and supplies the hydrogen to the fuel cell. The vaporizer produces a larger amount of vapor than is required to produce the requisite amount of hydrogen for the fuel cell to output a predetermined amount of energy.
Accordingly, it is possible to readily supply sufficient hydrogen to the fuel cell. Thus, the fuel cell can generate the required energy for driving the vehicle in a short time.